When receiving a new message, you are informed depending on the situation and setting.
If the app is selected, you are informed in that way:
A number is displayed next to the menu entry in the menu bar. The number indicates the number of new messages.
There is a sound notification. You can configure or deactivate the incoming message tone in the Settings.
A notification window opens. You can enable or disable the notification window in the settings.
If you selected , a sound is only played if a message comes in for conversations other than the current one.